Question 341 of 2,152

IPv6 First Hop Security →mediumMultiple ChoiceObjective-mapped

300-410 IPv6 First Hop Security Practice Question

This 300-410 practice question tests your understanding of ipv6 first hop security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

A network engineer is troubleshooting an IPv6 neighbor discovery issue on a switch running IOS-XE. Hosts on VLAN 100 are intermittently losing connectivity to the default gateway. The switch is configured with IPv6 First Hop Security features including RA Guard and DHCPv6 Guard. The engineer notices that the switch is dropping valid Router Advertisements from the legitimate router. What is the most likely cause of this issue?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The RA Guard policy is configured with 'device-role router' on the port connected to the legitimate router, but the router's MAC address is not in the allowed list.
Correct because RA Guard requires explicit authorization of routers; if the legitimate router's MAC is not allowed, its RAs are dropped.
B
DHCPv6 Guard is blocking DHCPv6 Advertise messages from the router, preventing hosts from obtaining IPv6 addresses.
Why wrong: Incorrect because DHCPv6 Guard blocks DHCPv6 server messages, not RAs; the symptom is about RA loss, not DHCP.
C
IPv6 Source Guard is dropping packets from the router because the router's IPv6 address is not in the binding table.
Why wrong: Incorrect because IPv6 Source Guard filters traffic based on source IPv6 address and MAC, but RAs are multicast and typically not filtered by Source Guard.
D
The switch has IPv6 unicast-routing enabled, causing it to send its own RAs and override the legitimate router.
Why wrong: Incorrect because the switch would send RAs only if it is configured as a router; the issue is about dropping RAs, not sending conflicting ones.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The RA Guard policy is configured with 'device-role router' on the port connected to the legitimate router, but the router's MAC address is not in the allowed list.

The RA Guard policy with 'device-role router' on the port connected to the legitimate router expects the router's MAC address to be in an allowed list. If the MAC address is not explicitly permitted, the switch drops all Router Advertisements from that port, including valid ones. This causes hosts on VLAN 100 to lose connectivity to the default gateway because they never receive the necessary RA to learn the gateway's IPv6 address and on-link prefix.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
The RA Guard policy is configured with 'device-role router' on the port connected to the legitimate router, but the router's MAC address is not in the allowed list.
Why this is correct
Correct because RA Guard requires explicit authorization of routers; if the legitimate router's MAC is not allowed, its RAs are dropped.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
DHCPv6 Guard is blocking DHCPv6 Advertise messages from the router, preventing hosts from obtaining IPv6 addresses.
Why it's wrong here
Incorrect because DHCPv6 Guard blocks DHCPv6 server messages, not RAs; the symptom is about RA loss, not DHCP.
✗
IPv6 Source Guard is dropping packets from the router because the router's IPv6 address is not in the binding table.
Why it's wrong here
Incorrect because IPv6 Source Guard filters traffic based on source IPv6 address and MAC, but RAs are multicast and typically not filtered by Source Guard.
✗
The switch has IPv6 unicast-routing enabled, causing it to send its own RAs and override the legitimate router.
Why it's wrong here
Incorrect because the switch would send RAs only if it is configured as a router; the issue is about dropping RAs, not sending conflicting ones.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the nuance that RA Guard with 'device-role router' does not automatically trust all routers; it requires explicit MAC address matching, and candidates may mistakenly think that setting the device role to 'router' alone is sufficient to permit RAs.

Detailed technical explanation

How to think about this question

RA Guard (RFC 6105) uses a policy that can be applied to switch ports to filter Router Advertisements based on device roles. When 'device-role router' is configured, the switch expects the sending device to be a router, but it also requires the router's MAC address to be in an allowed list (configured via 'match mac-address' under the RA Guard policy). If the MAC address is missing, the RA is dropped. This is a common misconfiguration where the policy is too restrictive, blocking legitimate RAs from the actual default gateway.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

Visual reference

Quick reference

Access Control Model Comparison

Model	Acronym	Who Controls Access?	Best For
Discretionary Access Control	DAC	Resource owner	Small teams, file shares
Mandatory Access Control	MAC	System / security labels	Classified govt / military
Role-Based Access Control	RBAC	Administrator (via roles)	Enterprise environments
Attribute-Based Access Control	ABAC	Policy engine (user + resource attributes)	Fine-grained, dynamic policies
Rule-Based Access Control	RuBAC	System rules / ACLs	Firewall rules, network ACLs

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 300-410 question test?

IPv6 First Hop Security — This question tests IPv6 First Hop Security — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The RA Guard policy is configured with 'device-role router' on the port connected to the legitimate router, but the router's MAC address is not in the allowed list. — The RA Guard policy with 'device-role router' on the port connected to the legitimate router expects the router's MAC address to be in an allowed list. If the MAC address is not explicitly permitted, the switch drops all Router Advertisements from that port, including valid ones. This causes hosts on VLAN 100 to lose connectivity to the default gateway because they never receive the necessary RA to learn the gateway's IPv6 address and on-link prefix.

What should I do if I get this 300-410 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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